This invention relates to a process for preparing substantially spherical particles of crosslinked thermosetting resin in a size range suitable for use as a friction particle. Also, disclosed is a novel polyester friction particle prepared according to this process or prepared according to a conventional process.
As used herein "friction particle" is intended to mean a particulate material having the properties of no substantial softening at elevated temperatures and a material which will not flow together or cohere with other particles, as would be the case with a "friction binder." A "friction particle" will not fuse with like friction particles, and is insoluble. A friction particle is held in place with a friction binder.
As used herein, a "friction binder" has the properties of flowability and adhesive and cohesive binding action, for the purpose of binding together the asbestos and other additives (including a friction particle) necessary for building a brake lining or other similar article of manufacture.
As used herein, a "friction element" is a composition useful as linings or facings in brakes, main clutches, and banded clutch facings of power transmission speed control structures of power driven devices such as automotive vehicles. Friction elements as now made are composed in general of the filamentous reinforcing friction material, bonded with a friction binder and containing other organic or mineral friction controlling agents which may include, by way of example, extending materials for imparting specific properties or characteristics to such friction elements, for example, heat resistance, resistance to moisture sensitivity, wear and noise. One such friction controlling agent is a friction particle as defined above.
It is known that friction elements which are intended for heavy duty brake or clutch operations in motor vehicles must withstand severe service conditions. In use they are subjected to rigorous treatment by repeated and often prolonged braking or clutching applications which develop high temperatures, particularly above 500.degree. F in the friction elements, and temperatures frequently exceed 1000.degree. F on the friction surface of the elements, while progressively decreasing inwardly of such surface. These high temperatures, especially when occurring during high speed stops or following repeated applications of the brakes, tend to depolymerize or otherwise decompose the substances which are employed as friction particles in the friction elements. Decomposition of the friction particles results in the formation of gaseous or liquid products of heat decomposition which can in some cases cause marked softening of the friction element with consequent loss of braking efficiency. When friction elements exhibit a loss of stability of friction characteristics originally existing, they can produce after vigorous braking applications a condition which automotive engineers customarily refer to as "brake lining fade." Friction particles which exhibit minimal loss of stability of friction characteristics in a friction element are said to have good friction stability and good braking fade resistance.
A friction element comprising a filamentous reinforcing friction material such as asbestos fiber, a friction binder, friction particle and other additives is heated to about 300.degree. to 400.degree. F and pressed at about 500 to 3000 lbs./in. in order to form a brake lining composition, clutch facing or other braking device. Thus the friction particle is substantially insoluble and infusible, softening only at elevated temperatures (i.e. above about 400.degree. to 500.degree. F).